1. Field of the Invention
This invention relates to an image sensing apparatus which uses an electronic flash device.
2. Description of the Related Art
Image sensing apparatuses arranged to record still pictures of photographing objects have recently come to be used either as electronic still video cameras or for industrial purposes. The camera of this kind employs a solid-state image sensor as an image sensing device to meet such requirements as a compact size, a light weight and a high reliability.
The solid-state image sensor, however, has a disadvantage which lies in a phenomenon called blooming. This phenomenon takes place as follows: When a part of an image sensing area receives incident light of strong intensity, the amount of electric charge generated there exceeds the charge accumulating capacity of the picture elements of the part and overflows into an adjacent part having a less quantity of incident light. This results in a false signal. In a known method for preventing this, picture elements are provided with an antiblooming gate for removal of excessive electric charge caused by electron-hole recombination. The solid-state image sensor which is arranged in this manner includes a frame-transfer type virtual phase CCD (charge-coupled device) which has the antiblooming gate arranged to discharge unnecessary electric charge. The mechanism of the antiblooming gate is as described below:
FIG. 1(a) of the accompanying drawings is a sectional view taken in the direction of transfer showing the virtual phase CCD having the antiblooming gate. Each picture element consists of one transfer gate area and one virtual phase area. The antiblooming gate 1 is disposed in an intermediate position between a virtual barrier and a virtual well within the virtual phase area. The illustration includes a transfer gate electrode 2; an N type impurity layer 3; and a P.sup.+ impurity layer 4. FIG. 1(b) shows potential distribution obtained in an electric charge accumulating mode. A clock signal of an arbitrary frequency is applied to the antiblooming gate 1.
FIG. 2 shows distribution of potential obtained in the direction of depth below the antiblooming gate 1. The distribution is obtained when a high level voltage is applied to the antiblooming gate 1. Under this condition, an electric charge signal which is an electric charge generated by photoelectric conversion in the electric charge accumulating mode comes to overflow an interface when it exceeds a capacity determined by a surface potential and the maximum value of a channel potential. The overflowing electric charge is then trapped on a surface level. Then, if the potential level of the antiblooming gate 1 is lowered in such a way as to obtained a pinning state, the surface is filled with holes with the P.sup.+ layer 4 of the channel stop area and that of the virtual gate area serving as supply sources. Under this condition, the electrons are recombined with holes on the surface level. The blooming which results from an excess of electric charge can be prevented with this action repeated during the process of accumulating the electric charge.
FIG. 3 shows the arrangement of the conventional electronic still video camera. The illustration includes a lens 11; a half-opening type shutter 12; a solid-state image sensor 13 which is a CCD or the like having the above-stated antiblooming gate; a driving circuit 16 which is arranged to generate a gate pulse for the solid-state image sensor 13; a light measuring circuit 14 which is provided for determining an apposite exposure; a color measuring circuit 15 for determining white balance; a system control circuit 17 which is arranged to generate timing pulses and control signals for the operation of the whole camera; an image signal processing circuit 18 which is arranged to create, from the signal output from the solid-state image sensor 13, color difference signals (R-Y and B-Y) and a luminance signal with a synchronizing signal (Y+Sync); a modulation circuit 19 which frequency-modulates the signals output from the image signal processing circuit 18; a recording amplifier 20 which amplifies the signal output from the modulation circuit 19; a magnetic head 21 which electromagnetically converts the signal output from the recording amplifier 20; a magnetic sheet (or disc) 22 on which magnetic signals are recorded by the magnetic head 21; a motor 23 which rotates the magnetic sheet 22; a servo circuit 24 which controls the rotation of the motor 23; and an electronic flash device 25 (hereinafter referred to as a flash device) which is used for illuminating an object to be photographed and is either incorporated within the camera or removably mounted on the camera. FIG. 4 is a timing chart showing the operation of the electronic still video camera of FIG. 3 performed for flash photography.
Referring to FIG. 4 along with FIG. 3, the conventional electronic still video camera performs a flash photographing operation in a manner as described below:
A pulse .phi.SS1 is produced as an instruction for opening the half-opening type shutter 12. In response to this instruction, the shutter 12 begins to operate. After the lapse of the delay time of an electrical signal due to a mechanical arrangement, the shutter 12 begins to open to give a desired aperture diameter. A clock signal .phi.AB of a frequency f0 Hz which prevents occurrence of blooming in an object's image due to the flash light of the flash device at the time of flashing of the flash device is applied to the antiblooming gate 1 either at the same time as the generation of the pulse .phi.SS1 or after the lapse of the estimated delay time of the shutter 12. A pulse .phi.SS2 is generated the instance the opening of the shutter 12 reaches the desired aperture diameter. In response to the pulse .phi.SS2, the shutter 12 is stopped from opening and is kept in the aperture position. The instant the aperture of the shutter 12 is stopped at the desired aperture diameter position, the flash device 25 is instructed to flash. In response to this, the flash device 25 flashes at a flashing time of at least 1/1000 sec. After flashing, a pulse .phi.SS3 which is an instruction for closing is applied to the shutter 12. The shutter 12 begins to close. After closing of the shutter 12, the accumulated electric charge of the solid-state image sensor 13 is read out to be subjected to a signal processing action. A signal thus obtained is modulated, amplified and then recorded on the magnetic sheet 22.
In accordance with the conventional arrangement, however, the attempt to obtain a sufficient antiblooming capability results in an extremely high degree of dark current, which degrades the picture quality.
The details of this are as follows: The antiblooming gate operates in the manner as described above. The surface pinning process of the operation is followed by a process during which holes are coming back to the channel stop area and the virtual gate area of the P.sup.+ layer which are the supply sources of the holes. Immediately after the bias of the antiblooming gate changes to a positive bias from a level at which the surface potential below the gate is pinned, most of the holes remain below the gate. After that, since the gate bias is high, there obtains a large fringing field between the edge of the antiblooming gate and that of the virtual gate area or that of the channel stop area. The holes remaining within the antiblooming gate area come along this filed back to the channel stop area and the virtual gate area which are in the state of P.sup.+. However, since the fringing field is large, the holes become hot holes. This brings about ionization by collision. Electrons resulting from this are collected as a false signal. Hereinafter, the dark current generated by this mechanism will be called a hot hole dark current.
With the solid-state image sensor having the antiblooming gate arranged in this manner, the amount of removal of the excess electric charge and that of the false signal generation can be determined by the one cycle of a clock signal for the antiblooming gate. With the accumulating time assumed to be unvarying, the antiblooming capability and the dark current thus can be considered proportional to the clock signal frequency.
In the flash photography, the flashing time of a discharge tube (xenon discharge lamp) which is used for generation of a flash light is very short. Therefore, the quantity of light emitted during this short time is very large. Since the antiblooming capability of the solid-state image sensor provided with the antiblooming gate, as mentioned above, is determined by the excessive electric charge removing amount within such a short period of time, a clock signal of a very high frequency must be applied to the antiblooming gate in the event of flash photography. As a result, the amount of dark current increases to deteriorate the picture quality.
Further, the conventional apparatus described above is arranged to discharge the electric charge of the solid-state image sensor before the shutter begins to open. Therefore, the solid-state image sensor is in its accumulating mode for a much longer time than the flashing time actually required by the image sensor for its photoelectric converting action. Besides, this allows the image sensor to receive a light signal during a period of time other than the flashing time. The quantity of incident light other than the flash light increases with the time required between the beginning and end of the shutter opening process. This presents two problems:
A first problem resides in picture quality deterioration due to a dark current and white point flaws. The dark current and the white point flaws are undesirable for a solid-state image sensor. The dark current results from generation of electric charge due to heat or from an ionization impact due to hot holes. The former is proportional to heat and time. The latter is proportional to the number of pulses applied to the gate. The white point flaw is caused by a difference in dark current generation between picture elements. A picture element having the dark current generated to a much greater degree than others gives the white point flaw. The seriousness of these problems increases to lower the picture quality accordingly as the accumulation time increases.
A second problem lies in the incidence of light on the solid-state image sensor before and after emission of the flash light. The quantity of such light incident on the image sensor during the periods other than the flashing time becomes an error of exposure light quantity. The rate of this error varies with the brightness of the object to be photographed. For example, the incident light obtained during the non-flashing time causes a very little exposure error to allow an apposite exposure in a very dark place. In a place which is not so dark, however, the light incident on the image sensor during the non-flashing time increases to cause an over-exposure. The long exposure time extending outside the flashing time thus makes control over the exposure light quantity of the image sensor difficult. In the case of the solid-state image sensor in particular, the latitude allowed to the exposure is narrow. In view of this, a shutter is sometimes arranged to permit control over the curtain speed thereof. However, the shutter of that kind requires a longer period of time before opening it and thus the above-stated problem becomes more conspicuous.
Further, the conventional apparatus of the above-stated kind is arranged to allow the solid-state image sensor 13 to perform its accumulating action over a period of time which is determined including the delay time of the operation of the mechanical half-opening type shutter 12, the operation time difference between individual shutters resulting from varied degrees of mechanical precision, inconstancy of the operating speeds of a shutter driving motor and electromagnets due to voltage variations, etc. As a result, the accumulating time of the image sensor 13 becomes very long.
While the solid-state image sensor 13 of the above-stated kind is known to have the drawbacks including the dark current and the white point flaw which is called a fixed pattern noise, the drawbacks increase in proportion to the accumulating time of the solid-state image sensor 13. The dark current and the white point flaws come to excessively degrade the output image of the image sensor when the amount of the electric charge exceeds a given value. Whereas, as mentioned above, the accumulating time of the solid-state image sensor is generally very long in the case of the conventional apparatus using the mechanical shutter 12. As a result, the output image either gives a coarse impression due to the dark current and the white point flaw or has white points appearing everywhere in the image to degrade the picture quality and to give a disagreeable picture.